java.lang.StringIndexOutOfBoundsException: length=0; index=0

/*Device Info*/
[硬]设备制造商=Genymotion
[硬]设备型号=Custom
[硬]CPU型号=null
[硬]RAM=1.04 GB
[硬]ROM=13.38 GB
[OS][三方定制系统版本]=Android/vbox86p/vbox86p:8.0.0/OPR6.170623.017/32:userdebug/test-keys
[OS]产品名称(adb shell)=vbox86p
[OS][系统名称]=Android
[OS][Android版本]=8.0.0(26)
[app]versionName=1.0
[app]versionCode=1
/*Exception*/
java.lang.StringIndexOutOfBoundsException: length=0; index=0
    at java.lang.String.charAt(Native Method)
    at io.github.TomGarden.tetris.CustomWeights.SettingActivity.TwoClickPreference.setBtn(TwoClickPreference.java:247)
    at io.github.TomGarden.tetris.CustomWeights.SettingActivity.TwoClickPreference.access$300(TwoClickPreference.java:39)
    at io.github.TomGarden.tetris.CustomWeights.SettingActivity.TwoClickPreference$2.run(TwoClickPreference.java:169)
    at android.os.Handler.handleCallback(Handler.java:789)
    at android.os.Handler.dispatchMessage(Handler.java:98)
    at android.os.Looper.loop(Looper.java:164)
    at android.app.ActivityThread.main(ActivityThread.java:6541)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)

TomGarden / Tetris

java.lang.StringIndexOutOfBoundsException: length=0; index=0 #30